Most semiconductor fabricating machines has conventionally incorporated the linear motion guide units therein for handling or transfer requirements. The linear motion guide units are sometimes forced to work in high temperature and vacuum atmosphere, and usually comprised of more than one rolling element, a guide member made thereon a raceway groove to allow the rolling element to roll on through there, and a slider fit over the guide member for linear movement relative to the guide member by virtue of the rolling elements so that the slider together with any moveable part carried on the slider is allowed to move relatively to the guide member. With the linear motion guide units constructed as stated just earlier, the rolling elements constantly come into direct contact with both the guide member and the slider, and therefore better lubrication is needed to make certain of steady operation over a long-term service period.
The advanced linear motion guide units have recently become required to meet with needs of maintenance-free operation. The linear motion guide units especially working on the semiconductor fabricating machines in clean environment are strongly expected to be responsible for little or no dirt.
A linear motion guide unit to cope with the aspects as stated earlier is disclosed in, for example Japanese Patent Laid-Open No. 2001-82469, in which a sleeve fits in a lengthwise bore in the slider to provide a non-loaded return passage for the rolling elements, the sleeve being of cellular texture made by first sintering finely powdery ultrahigh molecular weight polyethylene and subsequently filling in cells or voids in the resulting sintered product with any lubricant through dipping process.
With the technical development applied to the linear motion guide unit as stated just earlier, or dipping approach to immerse the sleeve of cellular texture in lubricant, nevertheless, lubricant is prone to adhere to only any or both the inside and outside circular surface of the sleeve rather than filling enough in pores or cells in the cellular texture of the sleeve, thereby resulting in the provision of the sleeve that is insufficient in impregnation of lubricant. Using the imperfect sleeve as stated earlier in the linear motion guide unit especially working in vacuum environment would pose a difficult problem that the linear motion guide unit could not be ensured against little or no dirt and also better lubrication. Polyethylene to make the sleeve has another disadvantage of less in heat resistance.